Spectrum spreading allows for multiple propagation paths between the transmitter and the receiver, which achieves an appreciable reception diversity gain.
One receiver conventionally used for this purpose is the “rake” receiver, which comprises a number of fingers operating in parallel to estimate the digital symbols transmitted. The reception diversity gain is the result of combining estimates obtained in the various fingers of the receiver.
In a spread spectrum CDMA system, the symbols transmitted, which are generally binary symbols (±1) or quaternary symbols (±1 ±j), are multiplied by spreading codes comprising samples, called chips, whose timing rate is higher than that of the symbols by a ratio called the spreading factor. Orthogonal or quasi-orthogonal spreading codes are assigned to different channels sharing the same carrier frequency, to enable each receiver to detect the sequence of symbols that is addressed to it by multiplying the received signal by the corresponding spreading code.
In cellular systems, the fixed transceiver serving a given cell generally transmits a beacon signal on a pilot channel which is assigned a particular pilot spreading code. The spreading code is communicated to the mobile terminals in or near the cell by means of system information broadcast by the base stations. The terminals measure the power at which the pertinent pilot codes are received. These measurements enable mobiles on standby to identify the best cell to use for random access, should this be necessary. They also identify, during a call, the cell or cells offering the best radio link conditions for a handover (intercellular call transfer), should this be necessary.
One feature of spread spectrum CDMA systems is the ability to support a macrodiversity mode. Macrodiversity consists in enabling a mobile terminal to communicate simultaneously with different fixed transceivers of an active set. In the downlink, the mobile terminal receives the same information several times. In-the uplink, the radio signal transmitted by the mobile terminal is picked up by the fixed transceivers of the active set to produce different estimates which are then combined in the network.
By combining different observations of the same information, macrodiversity achieves an improvement in reception that enhances the performance of the system.
It also enables soft handover (SHO) if the mobile terminal is moving around.
In a CDMA system, such as the UMTS, the power transmitted over the radio interface is adjusted by a control procedure in which the receiver sends transmission power commands (TPC) back to the transmitter with the aim of achieving target reception conditions. The transmission power commands consist of bits sent at a relatively high rate and whose value indicates if the transmission power must be increased or reduced.
In the case of a macrodiversity call, the fixed transceivers of the active set receive identical TPC bits from the mobile terminal. Respective corrective terms may be taken into account by the fixed transceivers to balance the power transmitted. For the same total power transmitted relative to a logical channel, i.e. for a given level of interference, the balancing of the powers transmitted is not optimized in terms of the error rate observed in the symbols transmitted.
An object of the present invention is to provide a more satisfactory solution.